1. Field of the Invention
The present invention discloses the details of robust micromachining devices for measuring mass flow rate in a gas or liquid medium according to the preamble of the independent claims. It concerns mass flow sensor in a micro-structure forms which are made of micromachining approach and methods of manufacture. The present micromachining flow sensor is built on a suspending membrane based on polyimide materials.
Unlike the conventional micro-fabricated devices were generally susceptible with the disadvantages of vulnerability and fragility in an inhospitable surroundings, the present invention provides robust micromachining devices with capabilities to operate under complementary harsh environments.
2. Description of the Related Art
Various thermal mass flow meters have been heretofore developed and commercially available on the market. Over the past few years, the advancement of micromachining technology has facilitated the realization of thermal mass flow sensors fabricated directly on silicon. The micromachining thermal mass flow sensors can offer the benefits of smaller size, lower power consumption, and higher reliability at lower cost compared to the conventional thermal mass flow sensors. In particular with the property of low power dissipation, micromachining thermal mass flow sensors could broaden its exploitation on many battery-operated applications.
The active area for flow sensing element on a conventional micromachining thermal mass flow sensor is typically fabricated on a thin membrane from silicon-based thin films, such as silicon nitride or silicon dioxide materials, which generally have a cavity underneath to provide good heat isolation. Unfortunately, the membrane made of silicon-based thin films is typically very fragile and vulnerable in dirty fluid channels with the existence of smoke or dust. The alien particles or debris in flow channels easily damage the membranes and cause the malfunction of sensors. Such drawbacks have significantly limited the micromachining flow sensor application in many industrial fields. It would, therefore, be desirable to develop a micromachining mass flow sensor which would not only endure the above mentioned problems but still heir the advantages from micro-structure forms.
The U.S. Pat. No. 6,184,773 B1 (Rugged Fluid Flow and Property Microsensor; by Ulrich Bonne et al.) revealed a microfabricated thermal flow sensor consisting of a back-etched honeycomb structures to support the membrane layer for device operation. Although the honeycomb structures provide a solution to improve the strength of suspending membrane, however the underneath honeycomb structure will lessen the thermal isolation of membrane, which could increase the power consumption and degrade the performance of devices. In addition, the membrane area above the space between honeycombs could still become the weakest points on the membrane layer; therefore those areas will possibly become the initial points of membrane breakage.
A microstructure flow sensor die with a Microbrick or microfill structure had been detailed in the U.S. Pat. No. 7,109,842 B1 (Robust Fluid Flow and Property Microsensor Made of Optimal Material; by Aravind Padmanabhan et al.). There are three embodiments revealed in the invention to realize the ideas of robust micro-sensor. The first one is to utilize a solid substrate having low thermal conductivity such as glass substrate. The second one is involving a micro-filling process of poor thermal conductivity material into the micro-cavity underneath the membrane. The third embodiment is to etch the glass substrate underneath the membrane into five circular openings. The above embodiments have various drawbacks such as high power consumption (first embodiment), enormous complexity of process, yield and cost issues (second and third embodiments).
The U.S. Pat. No. 7,040,160 (Flow Sensor; by Hans Artmann et al.) teaches a thermal flow sensor built on a region having poor heat conductivity, which is made of porous silicon or porous silicon dioxide on a silicon substrate. However, in order to attain good thermal isolation for the porous silicon region, the porous silicon layer should have enough thickness and high porosity. Since it is very time consuming and high cost to obtain such porous silicon layer, therefore it greatly reduces the feasibility of mass production. Moreover, to ensure the ambient temperature sensor having good thermal conductivity to the substrate is very crucial to prevent the temperature effect of thermal flow sensor. For the above reason, the ambient temperature sensor should not be disposed on the porous silicon region. Consequently, the porous silicon growth in selective area on silicon substrate becomes crucial to ensure the good performance of flow sensor.
Compared to the above mentioned inventions, the microfabricated thermal mass flow sensor in the current invention disclosed will present superior properties in many aspects include easiness of integration and fabrication, better liability, strength of membrane structure, and lower cost of manufacturing.